sarcomere and sliding filament theory

• Skeletal muscle fiber


  • Cylindrical cell, up to 30 cm long
  • Multiple peripheral nuclei
  • Many mitochondria
  • Glycosomes for glycogen storage
  • Myoglobin for O2 storage
  • Also contain myofibrils, sarcoplasmic reticulum, and T tubules
• Myofibrils


  Densely packed, rodlike elements that exhibit striations: perfectly aligned repeating series of dark A bands and light I bands
• Sarcomere


  Smallest contractile unit (functional unit) of a muscle fiber, the region of a myofibril between two successive Z discs, composed of thick and thin myofilaments made of contractile proteins
• Sarcomere


  • Thick filaments: run the entire length of an A band
  • Thin filaments: run the length of the I band and partway into the A band
  • Z disc: coin-shaped sheet of proteins that anchors the thin filaments and connects myofibrils to one another
  • H zone: lighter midregion where filaments do not overlap
  • M line: line of protein myomesin that holds adjacent thick filaments together
• Thick filament


  Composed of the protein myosin
• Thin filament


  Twisted double strand of fibrous protein F actin, with tropomyosin and troponin: regulatory proteins bound to actin
• Sarcoplasmic Reticulum (SR)


  • Network of smooth endoplasmic reticulum surrounding each myofibril, pairs of terminal cisternae form perpendicular cross channels
• T Tubules


  • Continuous with the sarcolemma, associate with the paired terminal cisternae to form triads that encircle each sarcomere
• Sliding Filament Model of Contraction


  1. In the relaxed state, thin and thick filaments overlap only slightly

  2. During contraction, myosin heads bind to actin, detach, and bind again, to propel the thin filaments toward the M line

  3. As H zones shorten and disappear, sarcomeres shorten, muscle cells shorten, and the whole muscle shortens
• Requirements for Skeletal Muscle Contraction

  1. Activation: neural stimulation at a neuromuscular junction

  2. Excitation-contraction coupling
• Events at the Neuromuscular Junction

  1. Nerve impulse arrives at axon terminal

  2. Acetylcholine (Ach) is released and binds with receptors on the sarcolemma

  3. Electrical events lead to the generation of an action potential
• Tendons are strong bands of connective tissue that attach muscle fibers to bone.
• Muscles are attached to bones by tendons.